How to Sim unlock an Iphone 5c?

I traded my Samsung Galaxy Note II for an Iphone 5c on craigslist. The Iphone that the man gave me has Sim lock on it. I went to AT&T, and they said they couldn't activate an account for me until it got unlocked. Any ideas on how to unlock the Sim without having to purchase something?

In this case the phone may not be a at&t phone at all, or the previous owner might have report the phone stolen, lost or the phone may be blacklisted. You can check esn at
checkesnfree.com
When you are at the website, select imei blacklist. You need to get the imdi off your iPhone(go to setting>general>about then scroll down to imdi. Type in the imei then it will say blacklisted or not blacklisted. If its blacklisted you got ripped off. You should have meet the person at at&t store before you made the trade. Or at least check the esn. You can ask the person to trade back phone, but mostly likely they won't reply back. So take this as a lesson.